In general, a shaft of an environmental control system pack air cycle machine of an air conditioning system must maintain an air cycle machine speed above a certain minimum value due to bearing capacity limitations of the environmental control system pack air cycle machine. For example, during altitude operation, current packs supply a flow of air to turbines of the packs to meet minimum value requirements. However, the supply of the flow of air to the turbines results in a pack flow restriction.
Because low pressure environmental control system architecture concepts desire minimum pack flow restrictions, what is needed is an alternative mechanism for maintaining a shaft speed of the environmental control system pack air cycle machine above a certain minimum value during altitude operation, where available bleed pressures are low.